MMP9 and neoplasm: Through dimensional reduction clustering analysis and cell type annotation of single-cell RNA sequencing data (Figures 2A, B), we identified five major cell populations in the OS microenvironment: (1) OS cells expressing COL1A1, LUM, DCN, RUNX2, and CDH11; (2) myeloid cells with high expression of CD74, CD14, and FCGR3A; (3) osteoclasts specifically expressing MMP9 and CTSK; (4) endothelial cells enriched for PECAM1 and VWF expression; and (5) tumor-infiltrating lymphocytes expressing CD3D, NKG7, and IL7R.